In order to join or fix various kinds of electronic parts to each other or other parts, there are used adhesives and/or insulating tapes having an adhesive layer. In the fabrication process of semiconductors, adhesives are used in adhesion of die bonding and potting or of packaging case and heat sink in IC elements or in insulating tapes for fixing lead frames, or the like.
With the recent increase of needs to make electronic parts high-performance, high-reliable, and small in size and light in weight, the adhesion of the electronic parts has come to be required not only to join and/or fix the electronic parts, but also to provide functions such as insulation, proofness to moisture and resistance to heat and reliability. Adhesives used in semiconductor devices also have come to be required to have higher electrical insulating property and adhesive strength as semiconductor elements have been made high in integration degree and/or density. High heat resistance is also required in many cases. In particular, insulating tapes for electronic parts such as insulating tapes for fixing lead frames are required to be good in current-leaking tendency, adhesive strength right after taping, adhesive strength after heat history and the like.
However, conventional adhesives for electronic parts involve a problem of reliability in that they are insufficient in heat resistance upon wire bonding, or corrosion of aluminum wirings and/or reduction in insulating property are caused by trace impurities contained therein.
As rubber-based adhesives for electronic parts, there have heretofore been used adhesives in which an unsaturated nitrile-conjugated diene copolymer rubber excellent in adhesive property to high-polar adherends is contained as a base polymer. Since the unsaturated nitrile-conjugated diene copolymer rubbers are weak in cohesion, a phenolic resin or epoxy resin is incorporated therein to offset their defects, thereby improving their adhesive property, workability, heat resistance, current-leaking tendency, etc. (for example, Japanese Patent Application Laid-Open Nos. 59681/1987, 246977/1987, 86494/1988, 60679/1989, 249876/1989, 292086/1989, 58885/1990, 91177/1990, 133483/1990, 167381/1990, 279781/1990 and 64387/1991, etc.).
Such unsaturated nitrile-conjugated diene copolymer rubbers have been generally prepared by an emulsion polymerization. The copolymer rubbers prepared by the emulsion polymerization however contain a great amount of residual impurities such as an emulsifier and metallic ions because an ionic surfactant is used as the emulsifier and the resulting polymer is solidified by a metallic compound after the polymerization. The use, as a base polymer for adhesives, of the copolymer rubber containing such ionic impurities involves problems of corrosiveness of the adhesives to metals and reduction of their electrical insulating property.
As a method of lowering the concentration of such residual impurities, it has been proposed to dissolve a copolymer rubber in a solvent and then pour the resulting solution into distilled water or the like to reprecipitate the copolymer rubber and dry it, or to purify a copolymer rubber under boiling in a poor solvent such as distilled water (Japanese Patent Application Laid-Open No. 279781/1990). However, such a purification process is very complicated and may be accompanied by a potential problem that the copolymer rubber is adversely modified due to deterioration or the like in the course of the purification process.
On the other hand, solution polymerization and bulk polymerization permit the reduction of the content of impurities such as metallic ions in the resulting copolymer rubber. However, these polymerization processes are generally difficult to provide an unsaturated nitrile-conjugated diene copolymer rubber as a high-molecular weight product. When the low-molecular weight copolymer rubber is used to prepare an adhesive composition, its flexibility-imparting and adhesive strength-holding properties and the like become insufficient.